A liquid crystal display element has advantages over other display element in terms of thinness, lightness in weight, and small power consumption. With the advantages the liquid crystal display element is widely used for image display devices such as a television and a monitor, and image display devices provided on OA (Office Automation) equipments, such as a word processor or a personal computer, or on information terminals such as a video camera, a digital camera, a mobile phone etc.
There are conventionally well-known liquid crystal display modes for the liquid crystal display element, such as the TN (twisted Nematic) mode using Nematic liquid crystal, the display mode using FLC (ferroelectric liquid crystal) or AFLC (anti-ferroelectric liquid crystal), and the polymer dispersed liquid crystal display mode. Another well-known mode is a IPS (In-Plane Switching) mode (horizontal electric field drive mode) which uses an electric field parallel to the substrate plane.
These liquid crystal display elements more quickly respond when driven by a high-voltage. Particularly, the liquid crystal display elements in the IPS mode tend to be driven by a high voltage not only for quick response but also for a larger aperture.
However, a general conventional liquid crystal display device uses a TFT (switching element) structure (circuit layout having a TFT), which is not suitable for high-voltage driving.
Here, the following explains a reason why the circuit layout of the conventional liquid crystal display device is not suitable for high-voltage driving. FIG. 17 is a cross sectional view showing a schematic structure of a display element 100 of a conventional IPS liquid crystal display device. As shown in the figure, the liquid crystal display element 100 are made of (i) two glass substrates (substrate 101 and substrate 102) and (i) liquid crystal (not shown). The liquid crystal is sealed in a dielectric material layer 103 between the two glass substrates. Further, on one of the surfaces of the substrates 101 facing the substrate 102, a signal electrode 104 and a counter electrode 105 are provided, facing one another, for applying a voltage between the dielectric material layers 103. Polarizers 106 and 107 are formed on the substrates 101 and 102, respectively, on their outward surfaces. To display an image, this liquid crystal display device applies a voltage between the two electrodes so that the generated electric field changes the orientations of their liquid crystal molecules.
FIG. 18 is a pixel equivalent circuit diagram illustrating a switching TFT provided in the foregoing liquid crystal display device. FIG. 19 is a schematic view showing a structure of a pixel of the display element 100. As shown in these figures, in the liquid crystal display device, the signal electrode 104 and the counter electrode 105 constitute an element capacitor Cp, and the signal electrode 104 is connected to a signal line S via the switching element TFT, and the counter electrode 105 is connected to a counter electrode line C. Further, the gate electrode of the switching element TFT is connected to the scanning line G. As shown in FIG. 19, the axes of the polarizers 106 and 107 respectively provided on the two substrates 101 and 102 are orthogonal to each other, and each form an about 45° angle with a direction in which the electrodes 104 and 105 are opposed (direction of electric field).
In this conventional liquid crystal display element, if assuming that the voltage applied to the counter electrode line C is expressed as DC (direct, constant), and the dynamic range of the acceptable voltage for the signal line S is expressed by Vpp, a voltage of ±Vpp/2 is applied to the display element 100 (element capacitor (element) Cp) if the display element 100 is stably driven by an alternating current. More specifically, when a voltage ranging from Vpp to Vpp/2 is applied to the signal line S while applying a constant voltage (Vpp/2) to the counter signal line C, a voltage of Vpp/2−0(=voltage of the signal line S−voltage of the counter electrode line C) is applied to the display element 100. Meanwhile, when a voltage ranging from Vpp/2 to 0 is applied to the signal line S, a voltage of 0−Vpp/2 is applied to the display element 100.
In such a liquid crystal display device, the drive voltage, i.e., the voltage applied to the display element 100, may be increased by applying an AC (alternating current) to the counter electrode line C, by, for example, changing the voltage applied to the counter electrode 105 (counter electrode line C) into an alternating current ranging from 0 to Vpp. In this case, if assuming that the voltage applied to the signal line S ranges from Vpp to 0, and the voltage applied to the counter electrode line C is 0, a voltage of Vpp−0 is applied to the display element 100. Meanwhile, if assuming that the voltage applied to the signal line S is Vpp−0, and the voltage applied to the counter electrode line C is Vpp, a voltage ranging from 0 to −Vpp is applied to the display element 100. In other words, by applying an alternating current ranging from 0 to −Vpp to the counter electrode line C, a voltage of 0−±Vpp is applied to the display element 100. The voltage applied to the display element 100 is thus doubled.
Note that, even when an AC is applied to the counter electrode line C, the drive voltage may not be sufficient. In this case, it is necessary to increase the dynamic range Vpp of the voltage applied from the signal line S. That is, in this case, the voltage applied to the display element 100 is increased by both application of alternating current to the counter electrode 15 and an increase in dynamic range Vpp of the voltage applied to the signal line S.
However, in this case, a great voltage is applied to the scanning line G, which decreases durability of the switching element TFT. The following explains this problem with reference to FIGS. 20 and 21. FIG. 20 is an equivalent circuit diagram of the conventional liquid crystal display device, showing two adjacent pixels (pixel 11 and pixel 12) connected to different scanning lines G. FIG. 21 is a timing chart showing an example of respective voltages of various sections in the pixels 11 and 12.
In FIG. 20, the pixel (display element) 11 and the pixel (display element) 12 having the structure of display element 100 shown in FIG. 18 are adjacently placed. In each of the element capacitor Cp of the pixels 11 and 12, one of the electrodes is connected to a common signal line S via the switching element TFT, while the other is connected to a common counter electrode line C.
In the structure of FIG. 21, when the switching element TFT of the pixel 11 is turned on under condition that the potential of counter electrode 105 (potential of counter electrode line C) is 0, and the signal voltage (voltage being applied to the signal line S) is Vpp, the potential of the drain D becomes Vpp, which is written into the pixel. Even if the switching element TFT is thereafter turned off, the potential difference between the drain D and the counter electrode line C (corresponding electrode) is kept at Vpp unless an opposite polarity is written to the pixel 12. In other words, the difference is maintained so long as the potential of the counter electrode line C is kept at 0.
However, when a opposite polarity is written to the pixel 12 which is adjacent to the pixel 11; in other words, when the switching element TFT is turned on under condition that the potential of counter electrode (potential of counter electrode line C) is Vpp, and the signal voltage is 0, the potential of the drain D of the pixel 11 becomes 2 Vpp. This is because the pixel 11 and the pixel 12 use the common counter electrode line C, and the potential difference between the two terminals of the element capacitor (pixel capacitor) is constant.
More specifically, when the potential of the counter electrode line C becomes Vpp so as to carry out writing to the pixel 12, the potential of one of the terminals (the one close to the counter electrode line C) of the element capacitor of the pixel 11 becomes Vpp since the pixel 11 and the pixel 12 use the common counter electrode line C (i.e., their counter electrode line C are unified). On the other hand, because the switching element of the pixel 11 is off, the accumulated charge is held in the element capacitor Cp, thereby making the potential difference between the two terminals of the element capacitor Cp constant. On this account, a change in potential of the terminal (the one close to the counter electrode line C) causes the same amount of change in potential of the terminal of the drain D.
Note that, as shown in FIG. 21, when the potential of the counter electrode line C becomes 0 so as to carry out writing of Vpp into the next pixel during when the switching element TFT is turned off after 0 is written to the pixel 12, the potential of the drain D of the pixel 12 decreases to −Vpp.
As described, in the circuit of the conventional liquid crystal display device, the potential of drain D greatly varies (to 2 Vpp or −Vpp in the foregoing example) from the writing potential (Vpp or 0 in the foregoing example).
Note that, to ensure accurate driving of all pixels, the switching element needs to be precisely turned on or off even in the presence of variation of drain potential (potential of the drain D). Accordingly, it is necessary to increase the difference between the voltage for turning on the switching element TFT (gate-on voltage) and the voltage for turning off the switching element TFT (gate-off voltage) by the same amount as that of variation of the drain potential.
However, an increase of potential applied to the scanning line G causes a great decrease of duration of the switching element TFT. Therefore, a significant damage is given to the switching element TFT particularly when the dynamic range Vpp of the voltage supplied to the signal line S is increased.
The drive voltage for the liquid crystal layer of the conventional liquid crystal display device is thus limited by the pressure resistance of the switching element TFT (thin film transistor). This limitation is particularly significant in a recently-developed polysilicon panel (e.g. monolithic polysilicon panel on which the pixels and the driving circuit are formed at once) which includes polysilicon TFTs as the switching elements. That is, the pressure resistance of the polysilicon TFT is generally low and therefore it is necessary to set a low upper limit of the drive voltage.
Such a circumstance raises a demand of a technique for ensuring large voltage application (drive voltage, electric field) to the liquid crystal in the liquid crystal display element (liquid crystal panel) using a switching element which is constituted of a TFT limited in durability.
A TN-mode liquid crystal display device, which is one of the various conventional liquid crystal display devices, has several defects, such as slow response, narrow viewing angle etc. to obtain a certain advantage over a CRT (cathode ray tube).
Further, tough the display mode using FLC or AFLC ensures quick response and wide viewing angle, it has serious defects in terms of shock-resistance, temperature characteristic etc., which keeps it from a wider application.
Further, the polymer dispersed liquid crystal display mode using light scattering does not require a polarizer, and is capable of high luminance display; however, this display mode is substantially incapable of viewing angle adjustment by a phase plate, and also has a defect in terms of response characteristic. The polymer dispersed liquid crystal display mode is therefore not considered significantly superior than the TN mode. In all of these display modes, liquid crystal molecules are aligned in a certain direction, and the vision differs depending on the viewing angle with respect to the liquid crystal molecules. That is, the effective viewing angle is restricted. Further, these display modes carry out display by causing the liquid crystal molecules to be rotated by way of electric field application, in this way the liquid crystal molecules are rotated while keeping the alignment state and therefore the response is slow. The display mode using FLC or AFLC has a certain advantage in terms of response speed, and viewing angle, but has a problem of nonreversible alignment destruction due to an external force.
Apart from the display elements using the molecule rotation due to the application of the electric field, an electronic polarization display mode using the electro-optic effect proportional to square of the electric field is proposed.
The term “electro-optic effect” indicates such a phenomenon that reflective index of a substance varies according to an outer electric field, and there are two types in the electro-optic effect: (i) the Pockels effect that is proportional to the electric field, and (ii) the Kerr effect that is proportional to square of the electric field. The Kerr effect, that is the Kerr electro-optic effect, was adopted early on in high-speed optical shutters, and has been practically used in special measuring instruments.
The Kerr effect was found by J. Kerr in 1875. Well-know materials showing the Kerr effect are organic liquid materials such as nitrobenzene, carbon disulfide, and the like. These materials are used for, for example, the aforementioned optical shutter, an optical modulating device, an optical polarizing device. For example, these materials are used, e.g., for measuring the strength of high electric field for power cables and the like, and similar uses.
Later on, Research has been conducted to utilize a large Kerr constant of the liquid crystal materials for use in light modulation devices, light deflection devices, and optical integrated circuits. It has been reported that a liquid crystal compound has a Kerr constant more than 200 times higher than that of nitrobenzene.
Under these circumstances, studies for using the Kerr effect to a display device have begun. It is expected that use of the Kerr effect attains a relatively low voltage driving than the Pockels effect that is proportional to electric field, because the Kerr effect is proportional to the square of the electric field. Further, it is expected that the utilization of the Kerr effect attains a high-response display apparatus because, e.g., the Kerr effect shows a response property of several p (micro) seconds to several m (milli) seconds, as its basic nature.
For example, disclosed by Tokukai 2001-249363 is a display device, as a display device using the Kerr effect, which includes: (i) a pair of substrates, at least one substrate of the substrates being transparent; (ii) a medium that is provided between the substrates, and that contains polar molecules in an isotropic phase state; (iii) a polarizing plate provided on an outer side of the at least one substrate; and (iv) electric field applying means for applying an electric field to the medium. However, the display device disclosed in the foregoing publication needs to be driven by a high drive voltage. Therefore, this display device cannot be driven by a TFT (thin film transistor, switching element) structure (circuit structure including TFT) of a conventional liquid crystal display device. More specifically, to drive a display device using the Kerr effect disclosed in the foregoing patent publication, a circuit suitable for high voltage driving is required, and the circuit also needs to be compatible with a TFT of a conventional liquid crystal display device.
The present invention is made in view of the foregoing conventional problem, and the object is to provide a display device which can be driven by a high voltage even with a switching element constituted of a TFT limited in durability.